An increasingly important package design in the surface mount technology area is the Ball grid array (BGA). BGA's offer many advantages over standard fine-pitch surface mount and pin grid array technologies. These advantages include reduced placement problems since BGAs are self-centering, reduced handling issues because there are no leads to damage, lower profile and higher interconnect density. However, significant drawbacks in BGA technology exist due to the inability to effectively test BGA packaged devices once they are configured for circuit insertion and once they are actually soldered into a circuit. In the latter instance it is especially important to be able to test the BGA device without disturbing its performance.
Electronic test instruments (e.g., oscilloscope, logic analyzer, emulator) are used to analyze various electrical aspects of integrated circuits (IC's) including voltage and current waveforms. Typically, a loaded printed circuit board is crowded with various electrical components, including multiple IC packages. Due to the close spacing of components on the board (i.e., high "board density") it is often difficult to electrically connect the IC's to the test instrument.
BGAs only make this problem worse since there are no "leads" to access for testing purposes, the balls on the BGA are small, and the balls are inaccessible. A modification of the BGA technology, the Plastic Ball Grid Array (PBGA), was made to aid in testing during fabrication. In this technology, the IC is attached to a substrate, typically a printed circuit board (PCB), which forms the bottom layer of the BGA. Traces on the PCB connect between the IC and test pads on the PCB which are large enough to be accessed by test probes connected in turn to test instruments. However, after the PBGA device has been given a final functional test, the outer portion of the PCB is sheared off leaving only a small remnant of these traces around the peripheral of the PBGA. Since most PBGA's are shipped to the customer in this condition, testing prior to insertion in a circuit and afterwards presents great difficulties. In particular, testing an inserted PBGA causes significant modification in circuit performance resulting in questionable test results.